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Mathematical models for predicting filter and facial leak penetration of dust respirators were described. The models were based on previously obtained experimental data on filter and leak performance of dust respirators against oleic-acid aerosols having particles with aerodynamic diameters of 0.14 to 11.34 microns. A spread sheet was written that combined the data on penetration versus aerodynamic diameter for filter and facial seal leaks at different pressure drops. The data were treated by a computerized regression program to yield equations that expressed percentage leak penetration as a function of particle diameter or particle diameter, pressure drop, and work rate. The equations predicted the data with a root mean square accuracy of 15.9 and 13.8 percent for dust, and fume and mist cartridges, respectively, and 19.2 percent for single/use respirators. A simplified linear equation that expressed percentage leak penetration as a function of the correction factor (CF) that depended on the work rate that the respirator was to be used and a factor that expressed the leakage as a fraction of the total flow rate under sedentary conditions (work rate equal to zero) was also derived. The equation could predict the experimental data with an accuracy of 25 percent. The authors conclude that the equations can predict respirator performance as a function of aerosol particle size for any work rate and leak penetration. The models offer a possible alternative to field measurements of performance.